Add OpenSSF best practices badge to the README

Add the OpenSSF best practices badge to the main README so we may show the
progress on the badge as we complete the passing level and work through silver
and gold levels.

docs/README.md

@@ -13,6 +13,7 @@ cascade:
 
 [![License](https://img.shields.io/badge/License-Apache%202.0-blue.svg)](https://github.com/tektoncd/dashboard/blob/main/LICENSE)
 [![Go Report Card](https://goreportcard.com/badge/tektoncd/dashboard)](https://goreportcard.com/report/tektoncd/dashboard)
+[![CII Best Practices](https://bestpractices.coreinfrastructure.org/projects/6543/badge)](https://bestpractices.coreinfrastructure.org/projects/6543)
 
 <p align="center">
   <img src="tekton-dashboard-color.svg" alt="Tekton Dashboard logo (Tekton cat inspecting resources with a magnifying glass)" width="200" />